Corrosion Factors

The factors listed earlier have been organized in a framework of six
categories with a number of subfactors as shown in the following
Table. According to
Staehle's materials degradation model, all engineering materials are reactive
and their strength is quantifiable, provided that all the variables involved in
a given situation are properly diagnosed and their interactions understood
[7].
The corrosion based design analysis (CBDA) approach was further developed from
the initial framework as a series of knowledge elicitation steps to guide
maintenance and inspection decisions on the basis on first principles [8].

Factors and contributing elements controlling the
incidence of a corrosion situation [7]

The two most important of these steps are described in the following
Figures for respectively the environment and the material definitions. Each of the
numbers in brackets in these Figures identifies an explicit action that needs to be
considered for each definition.

Analysis sequence for determining environment at a
location for analysis

Analysis sequence for determining materials at a
location for analysis (LA) matrix

A brief explanation of the individual elements in the environment definition follows:

"Nominal Chemistry" refers to the bulk chemistry. For
components exposed to ordinary air atmospheres, the "Major" elements mean
humid air. The "Minor" elements refer to industrial contaminants such as SO2
and NO2

"Prior Chemistry History" refers to exposures to
environmental species that might still reside on the surfaces or inside crevices

"System Sources" refers to those environments that do not
come directly from a component but from an outside source

"Transformations" refers, for example, to microbial actions
that can change relatively innocuous chemicals such as sulfates into very
corrosive sulfide species that may accelerate hydrogen entry and increase
corrosion rates

"Concentration" refers to accumulations much greater than
that in the bulk environment due to various actions of wetting and drying,
evaporation, potential gradients, and crevices actions that prevent dilution

"Inhibition" refers to actions taken to minimize corrosive
actions. This usually involves additions of oxygen scavengers or other chemicals
that interfere directly with the anodic or cathodic corrosion reactions.

The end point of the process is an input to a location for
analysis (LA) matrix that is illustrated inthe following
Figure for the locations in a
steam generator.

Schematic view of steam generator with different
locations for analysis

The LA template of the locations that correspond to most likely failure sites
along tubes in a steam generator of a pressurized water nuclear power plant is detailed in
the following Table for the main failure modes and sub-modes
considered in such analysis. Maintenance and inspection actions can be decided
upon by following developing trends monitored in each LA matrix thus produced.

The framework summarized above, which was initially developed to
predict the occurrence of stress corrosion cracking (SCC), was extended to other
corrosion modes/forms. Additionally, an empirical correlation was established
between the factors listed in Table of factors and the forms of corrosion described
earlier in the
previous Module. Recognized corrosion experts were invited to complete an
opinion poll listing the main sub-factors and the common forms of corrosion as
illustrated in the example shown in the following Figure. Background information on the
factors and forms of corrosion was attached to the survey. A total of sixteen
completed surveys were returned subsequently analyzed.

Opinion poll sheet for the most recognizable
forms of corrosion problems

The following Figure presents the Box-and Whisker plots of the results
obtained with pitting corrosion.When presented in this fashion, such
results can provide a useful spectrum of factor and sub-factor confidence
levels.

Box and whisker plots of the survey results
obtained for the factors and sub-factors underlying the appearance of pitting
corrosion

Propose some arguments to explain the high variance,
visible in the previous Figure, between expert opinions on the factors causing pitting corrosion.

Linking the corrosion factors with possible forms of corrosion in this
fashion may provide guidance to inexperienced corrosion failure investigators
who have typically limited knowledge of corrosion processes. A listing of the
most important factors should therefore help to increase the awareness of the
complexity and interaction of the variables behind most corrosion failures and
reveal how ‘experts’ have reduced such complexity to a reduced set of
variables, as the compiled results of the survey indicate in the following
Figure.

Results of compiled survey of corrosion experts
highlighting the most important correlations between corrosion forms and
factors

An application of the compiled framework could be to test one’s skills
against the ‘experts’ as illustrated in the following Figure.

Comparison of the answers of one expert with the
some of the compiled expert survey results

Another application of this practical correlation would be to use the
framework of factors vs. forms for archiving data in an orderly manner. Analysis
of numerous corrosion failure analysis reports has revealed that information on
important variables is often lacking [9]. The omission of important information from corrosion reports is
obviously not always an oversight by the professional author. In many cases, the
desirable information will simply not be (readily) available and require a
special investigation to be completed.